The Pilot Project studies will support the overall program entitled "Developmental and Translational Pharmacology of Pediatric Antimicrobial Therapy", submitted as an application responsive to RFA-HD-10-026: Specialized Center in Research in Pediatric Developmental Pharmacology (RPDP) Program (U54). The overall theme of the proposed program at the UC San Diego is to bring together non-clinical and clinical experts in the fields of developmental physiology, pharmacology, and infectious diseases to advance the field of pediatric developmental pharmacology. To promote the development of innovated research ideas and foster new collaborations, the UC San Diego RPDP Center includes a Pilot Project program. This program supports limited Pilot Projects for 1 or 2 years in duration to generate focused explorations related to existing RPDP Center Projects or to generate preliminary information for new Project applications. These Pilot Projects may originate from within UC San Diego or from other NICHD supported RPDP Centers. Administrative Core will organize the solicitation and selection process and the Scientific Advisory Committee members will assist the PIs and Core Directors with selection. The Pilot Projects that address general developmental pharmacologic issues not specific to antibacterial therapeutics may be considered but must involve either the Pharmacometric Core, the Quantitative Pharmacology Assay Core or both. Projects that promote interactions with other RPDP Centers, support junior investigators and have the potential to provide the scientific basis for larger clinical trials will be encouraged. Two Pilot Projects were selected from an initial pool of applicants for UC San Diego RPDP Center Years 1-2. They are entitled "Plasma and intracellular cathelicidin responses to infection in infants and young children" and "Developmental changes in aminopenicillin renal excretion in infants and young children" and are linked to Projects II and III, respectively. Both of these projects utilize the Pharmacometric and Quantitative Pharmacology Assay Cores and provide opportunities for trainee involvement, as well. In program Years 2-4, solicitations will be made for the Year 3-5 Pilot Projects.